2,6-Dichloropyridine is useful as a starting material for preparing 2,3,5,6-tetrachloropyridine. 2,3,5,6-Tetrachloropyridine is generally prepared by
chlorinating 2,6-dichloropyridine in a liquid phase in the presence of a metal halide as described in U.S. Pat. No. 3,538,100. This process provides 2,3,5,6-tetrachloropyridine at a purity as high as 96% or more without involving any purification procedure. Since the reaction successively proceeds, 2,3,6-trichloropyridine is also produced during the reaction, reaching a purity exceeding 85%. Therefore, this process is also admittedly excellent as a process for producing 2,3,6-trichloropyridine. However, satisfactory achievement of the process requires use of high-purity 2,6-dichloropyridine as a starting material. Impurities, e.g., pyridine and 2-chloropyridine, in the starting 2,6-dichloropyridine would result in disadvantages, such as formation of tar substances as by-products, reduction of product purity, reduction of reaction rate, and the like.
On the other hand, known processes for preparing 2,6-dichloropyridine include (1) a process comprising photo-chlorination of pyridine in a gaseous phase as taught in JP-B-52-3935, JP-B-52-3936, and JP-B-55-4742, and JP-A-60-78967 (the term "JP-B" as used herein means an "examined published Japanese patent application" and "JP-A" as used herein means an "unexamined published Japanese patent application") and (2) a process comprising photo-chlorination of 2-chloropyridine in a liquid phase as taught in U.S. Patent 3,557,124 and JP-B-55-4744.
The process (1) concomitantly produces 2-chloropyridine so that the yield of the desired 2,6-dichloropyridine is not always satisfactory. Besides, purification of the resulting product requires a number of stages including neutralization of the reaction product with an alkali, extraction with a solvent, and fractional distillation of the extract to separate the extracting solvent, pyridine, 2-chloropyridiene, and 2,6-dichloropyridine, thus attaining only a low yield from pyridine. The 2-chloropyridine formed as a by-product in the process (1) may be subjected to the process (2) to effectively obtain 2,6-dichloropyridine, but the process (2) not only needs an apparatus for photo reaction independently of the apparatus for photochlorination of pyridine but also causes additional problems, such as deposition of 2-chloropyridine hydrochloride onto a lamp.
It has hitherto been considered that the chlorination reaction of 2-chloropyridine in a liquid phase would be greatly retarded unless the reaction is carried out under light irradiation or in the presence of a catalyst. This is believed to arise from the fact that, with no catalyst being used, the liquid phase reaction has actually been conducted at a temperature lower than 160.degree. C and at atmospheric pressure because 2-chloropyridine has a boiling point of 170.degree. C. Under such reaction conditions, the reaction is slow and is also accompanied by formation of large quantities of by-products, e.g., 2,3-dichloropyridine and 2,5-dichloropyridine. In addition, the reaction concomitantly produces products of higher chlorination order, e.g., 2,3,5-trichloropyridine, 2,3,5,6-tetrachloropyridine, and pentachloropyridine.
On the other hand, in cases where the reaction is effected in the presence of a metal halide as a catalyst, the reaction is accelerated but still involves problems of formation, as a - by-product, of 2,3-dichloropyridine or 2,5-dichloropyridine and formation of chlorination products of higher order which is considered to accompany such side reactions.